This invention relates to a high-temperature sliding member and a method for manufacturing the sliding member for use in bearings and seals in high-temperature rotating machines such as steam turbines and gas turbines, or to a sliding member that is suitable for applications requiring wear resistance and low friction such as cutting tools.
Application of a ceramic coating to improve wear resistance and corrosion resistance of bearings and seals made of metal materials is widely practiced. Materials used for making such ceramic coatings include titanium nitrides (TiN), titanium carbides (TiC), chromium nitrides (CrN), boron nitrides (BN), and diamond-like carbon (DLC). Among these, TiN and CrN are already widely applied industrially as hard coatings on metal molds and cutting tools.
As conventional methods for making such hard coatings, following surface improvement techniques are on the table: ion plating method including physical vapor deposition (PVD) or chemical vapor deposition (CVD), sputter deposition, plasma CVD and ion implantation. In particular, dynamic ion beam mixing (DM) method, which combines the vapor deposition method with the ion implantation technology is receiving keen interest, because the coating can bond tightly to the substrate and the coatings can be produced at low processing temperatures.
One of the ceramic coatings that is widely in use is TiN, which is a typical substance forming an interstitial solid solution compound, is known to have a face-centered cubic crystalline structure. TiN has a NaCl type crystalline structure where nitrogen atoms enter in the lattices formed of Ti. The composition range of TiNx is as broad as 0.8 less than x less than 1.16, and when x is changed within this range, the lattice spacing of TiN is altered. Because of the superior resistance to wear and corrosion, TiN coating is also being used in bearings or seal components.
For application to rotating machines operating at high temperatures, such as steam turbines and gas turbines, there has been a need for hard coatings having a superior wear resistance, a high temperature corrosion resistance as well as superior high temperature sliding properties, as operating temperatures become higher in practice today. TiN coating is being considered for such applications, but it is known from experimental results to date that, because of insufficient corrosion resistance at high temperatures of TiN coating itself, durability of TiN coating has been in doubt when TiN coating is to be exposed to high-temperature air atmosphere or high-temperature steam. Therefore, the current state of art of TiN does not permit the use of TiN coating for such applications.
Also, for general purpose rotating machines such as pumps, there has been a tendency to increase rotational speed and operating pressure, resulting in a need for sliding components that can withstand severe operating conditions of high loads and high peripheral speeds. Conventional TiN coatings has become known to be inappropriate for such applications that present severe sliding conditions, because of inadequate hardness and wear resistance of TiN coating itself.
This invention was made to solve the problems outlined above, and an object is to provide a sliding member that can resist high-temperature corrosion while retaining the superior wear resistance and low frictional properties of TiN coating. Another object is to provide a sliding member having superior sliding properties to meet the needs of rotating machines operating at high rotational speeds and high pressures, by further improving the superior wear resistance and low frictional properties of TiN coating.
This invention relates to a sliding member comprising a substrate and a hard coating formed on the substrate, wherein the hard coating comprises a nitride-based material containing substantially TiN and at least one element selected from the group consisting of Al, Cr, Zr and Hf, and having a face-centered cubic crystalline structure with a lattice constant ranging from 0.414 to 0.423 nm in a crystal of the nitride-based material.
For the purpose of improving the resistance of TiN coatings to high temperature corrosion and oxidation, the inventors have been investigating technologies for obtaining nitride-based coatings containing elements other than Ti and N as well as methods for producing such products. That is, the primary object is to improve the resistance to high-temperature corrosion without losing the excellent sliding properties of TiN (wear resistance and low friction coefficient) by developing technologies for producing nitride-based thin films containing elements other than Ti and N. The result is a discovery that such a material has a face-centered cubic crystalline structure and contains substantially TiN and at least one element selected from the group containing Al, Cr, Zr and Hf, and that the lattice constant should be less than 0.423 nm, because if it exceeds this value, Vickers hardness becomes no more than 2000 and wear resistance becomes insufficient. These materials may also be used generally where sliding resistant properties are required.
It is suggested that a nitride-based material of this invention, which substantially comprises TiN but also containing at least one of Al, Cr, Zr and Hf, is a material in which some sites of Ti in a face-centered cubic crystalline structure is substituted by at least one of the elements selected from the group consisting of Al, Cr, Zr and Hf, and also has a face-centered cubic crystalline structure.
Investigations to date have demonstrated that the object is achieved when the nitride-based material has a face-centered cubic crystalline structure, the lattice constant is between 0.414 to 0.423 nm, the Vickers hardness of the material is not less than 2500 when the crystallite size of the nitride-based material is optimized, and has the following composition, excepting inevitable impurities such as carbon, oxygen, etc.
A preferable chemical composition of a sliding member made of the nitride-based material is defined in a formula, excepting inevitable impurities: Ti(100xe2x88x92x)Mex nitride compound, where Me represents at least one element selected from the group consisting of Al, Cr, Zr and Hf, and x is in a range given by a relation: 2 atomic % xe2x89xa6xxe2x89xa630 atomic %.
Such a member may be made by the DM method, which allows metallic elements, Ti and additives, to be vapor deposited on a substrate in a vacuum while implanting nitrogen ions into the deposit. This method enables to produce a coated product having the coating adhering tightly to the substrate in a relatively low temperature process. It is preferable that the substrate have a low coefficient of thermal expansion of not more than 11xc3x9710xe2x88x926 so as to produce tight bonding, which can be met by stainless steels, such as SUS420J2 or SUS630, or nickel-based alloys such as Incoloy 909.
It is preferable that the acceleration voltage for the ion beam be less than 40 kV, because a higher acceleration voltage requires a large sized acceleration device, leading to a higher processing cost and a need for radiation protection. On the other hand, if the acceleration voltage is less than 1 kV, coating does not bond tightly to the substrate so that the product is not suitable for high-temperature sliding applications.
The results of x-ray diffraction measurements suggest that the preferred crystallite size is several nm to 100 nm. Thickness of the hard coating may be adjusted for each application but it is preferable that the thickness be less than several tens of micrometers because of cost and residual stress considerations.
The proportion of additives during the process of making the hard coating using the DM method can be adjusted by controlling the evaporation rate of Ti and the additive elements respectively. The face-centered cubic crystalline structure of TiN is produced by entering of nitrogen atoms in the Ti lattice as interstitial solid solution. When one or more of the elements Al, Cr, Zr or Hf is added to TiN, the face-centered cubic crystalline structure of TiN becomes irregular as the concentration of the additive element increases, and ultimately reaches an amorphous state or attains other crystalline structures. Therefore, to retain wear resistance and lower coefficient of friction, it is preferable that the total concentration of additive elements be not more than 30 atomic %. Also, studies to date indicate that the resistance to high-temperature corrosion is increased as the concentration of the additive element is increased, but it is preferable that the lower limit of concentration be determined so as to enable customizing the product to application conditions, in terms of the severity of corrosion of high-temperature steam or high-temperature air.
It is preferable that the crystals be oriented to (111) planes. It is possible to orient the crystals to (111) planes during the DM method, by controlling the implantation conditions of the nitrogen ion beam such as, for example, acceleration voltage, current density, implantation energy (W/cm2), and the beam incidence angle.
On the other hand, the inventors have also developed a nitride-based material that is useable in sliding applications that do not demand high temperature strength. Such a sliding member comprises a substrate and a hard coating formed on the substrate, wherein the hard coating comprises a nitride-based material containing substantially TiN and at least one element selected from a group consisting of B and Si, and having a face-centered cubic crystalline structure comprising crystallites of an average size of not more than 9 nm. The process leading to such a concept is outlined below.
For the purpose of improving the hardness of TiN coatings and wear resistance, the inventors have been investigating technologies for obtaining nitride-based coatings containing elements other than Ti and N as well as methods for producing such products. That is, the primary object is to improve the hardness and wear resistance by developing technologies for producing nitride-based thin films containing elements other than Ti and N. The result is a discovery that such a material has a face-centered cubic crystalline structure and contains substantially TiN and at least one element selected from the group containing B and Si, and Vickers hardness is higher than 3000 when the crystallite size is not more than 9 nm, and has the following composition, excepting inevitable impurities such as carbon, oxygen, etc.
A sliding member made of a second group of nitride-based materials has a chemical composition defined in a formula, excepting inevitable impurities: Ti(100xe2x88x92x)Mex nitride compound where Me represents at least one element selected from the group consisting of B and Si, and x is in a range given by a relation: 2 atomic % xe2x89xa6xxe2x89xa630 atomic %.
As in the first group, such a member may be produced by the DM method, which allows metallic elements, Ti and additives, to be vapor deposited on a substrate in a vacuum while implanting nitrogen ions in the deposit. This method enables to produce a coated product having the coating adhering tightly to the substrate in a relatively low temperature process. It is preferable that the substrate have a low coefficient of thermal expansion of not more than 11xc3x9710xe2x88x926 so as to produce tight bonding, which can be met by stainless steels, such as SUS420J2 or SUS630, or nickel-based alloys such as Incoloy 909. The substrate may include other steel materials than the above referred. Also, for the purpose of making wear resistant parts or cutting tools, various ceramic materials such as Sic, Si3 N4 and Al2O3 as well as super-hard alloys such as WC may be used.
It is preferable that the acceleration voltage for the ion beam be less than 40 kV, because a higher acceleration voltage requires a large sized acceleration device, leading to higher processing cost and a need for radiation protection. On the other hand, if the acceleration voltage is less than 1 kV, coating does not bond tightly to the substrate, and the product is not suitable for sliding applications. Thickness of the hard coating may be adjusted for each application but it is preferable that the thickness be less than several tens of micrometers because of cost and residual stress considerations.
The proportion of additives during the process of making the hard coating using the DM method can be adjusted by controlling the evaporation rate of Ti and the additive elements. The face-centered cubic crystalline structure of TiN is produced by entering of nitrogen atoms in the Ti lattice as interstitial solid solution. When one or more of the elements B and Si is added to TiN, as the concentration of the additive element increases, face-centered cubic crystalline structure of TiN becomes irregular, and ultimately attains other crystalline structures. Therefore, to retain superior wear resistance and lower coefficient of friction, it is preferable that the total concentration of additive elements be not more than 30 atomic %. Also, studies to date indicate that the hardness and wear resistance are increased as the concentration of the additive element is increased, but it is preferable that the lower limit of concentration be determined so as to enable customizing the product to the severity of sliding conditions.
It is preferable that the crystals be oriented to (111) planes. It is possible to orient the crystals to (111) planes during the DM method, by controlling the implantation conditions of the nitrogen ion beam such as, for example, acceleration voltage, current density, implantation energy (W/cm2), and the beam incidence angle.
In the manufacturing process of the above described sliding members, it is permissible to form a hard coating on the substrate by simultaneously depositing in a vacuum Ti and at least one element selected from the group consisting of Al, Cr, Zr, Hf, B and Si on the substrate while irradiating the substrate with ion beams containing substantially nitrogen ions.
Another aspect of the present invention is a sliding mechanism comprising a combination of a movable member and a static member, wherein either the movable member or the static member is made of a sliding member according to any of claims 1 to 4 or made by a method according to claim 5, and a remaining member is made of a material containing carbon. The material containing carbon may be a material containing substantially carbon, a material infiltrated with carbon or a thin film containing carbon.
In a sliding member according to any of claims 1 to 4, a method according to claim 5 or a sliding mechanism according to claim 6 or 7, the substrate may be a metal material.
Another aspect of the present invention is a dressing tool comprising a sliding member according to any of claims 1 to 4, or comprising a sliding member made by a method according to claim 5.